1. Field of the Invention
The present invention relates to a piston for an internal combustion engine for an automobile or the like, and also to an internal combustion engine to which the piston is applied. More particularly, the present invention relates to an improvement in the shape of a depression formed in the top surface of a piston to maintain a tumble flow of intake air inside a cylinder.
2. Description of the Related Art
In conventional internal combustion engines (hereinafter occasionally referred simply to as “engines”) in which an air-fuel mixture is combusted in a combustion chamber to generate power, a tumble flow (vertical vortex flow) generated in a cylinder is effectively utilized to increase the combustion efficiency of fuel, and thus improves the output, the exhaust emission, the fuel consumption rate, and so forth. That is, the tumble flow agitates inside the cylinder to promote evaporation and atomization of fuel, achieving an excellent fuel property of fuel in the combustion chamber.
Various configurations have been proposed so far to positively generate such a tumble flow. For example, Japanese Patent Application Publications Nos. 9-105330 and 11-218026 (JP-A-9-105330 and JP-A-11-218026) disclose forming an intake port as a tumble port. That is, the flow line of intake air flowing into a cylinder is set closer to the vertical direction by setting the axis of the intake port closer to the vertical direction at a portion where it opens into the cylinder, allowing to obtain a large tumble flow.
Japanese Patent Application Publication No. 7-119472 (JP-A-7-119472) and
WO 00/77361 disclose providing a tumble control valve in an intake passage, which opens and closes to generate a large tumble flow in a cylinder as necessary. Specifically, the intake passage is partitioned by a separation wall (partition plate) into two, upper and lower flow paths, and a tumble control valve for opening and closing the lower flow path is provided. When it is necessary to generate a large tumble flow in the cylinder (for example when the engine is cold), the tumble control valve is closed and intake air is sent from only the upper flow path, so that the flow line of intake air flowing into the cylinder is set to a direction closer to the vertical direction, allowing to obtain a large tumble flow.
The engines in which means for positively generating a tumble flow such as described above is implemented adopt a piston formed with a depression for maintaining the tumble flow in its top surface (piston crown). For example, JP-A-9-105330 discloses forming a depression with a large length (the dimension in the direction along the tumble flow axis (the center line of a vortex flow)) in a region facing an exhaust valve, and forming a depression with a small length in a region facing an intake valve. JP-A-11-218026 discloses forming a circular bowl-shaped depression in the piston top surface. JP-A-7-119472 discloses forming a depression of which outer peripheral shape in the direction along the tumble flow axis is generally arcuate, and of which outer peripheral shape in the direction perpendicular to the tumble flow axis is straight, as viewed in plan. WO 00/77361 discloses forming in a piston top surface a depression which is generally rectangular or generally trapezoidal as viewed in plan. By forming such depressions in the piston top surface, a tumble flow is guided by the piston top surface to maintain the tumble flow.
The inventors of the present invention have found that the conventional depression shapes cannot achieve an optimum tumble flow, and thus have studied on the shape of a depression to be formed in the piston top surface. They also have considered the shape of a piston top surface more suitable to generate an effective tumble flow. A detailed description is as follows.
In the case where a depression is formed in a piston top surface to maintain a tumble flow, the tumble flow flows along the shape of the depression (the shape of the surface of a curved depression). Therefore, the outside diameter of the tumble flow is determined generally according to the width of the depression (the dimension in the direction perpendicular to the tumble flow axis (the center line of a vortex flow): the dimension in the direction in which an intake valve and an exhaust valve face each other).
A part of the tumble flow generated in the central region in the axial direction of the tumble flow (a part of the tumble flow generated at the center of the piston top surface, which is hereinafter referred to as “center tumble flow”) flows in a region where the depression width is large, and therefore is hardly influenced by a cylinder bore inner wall but flows along the shape of the depression. Therefore, a tumble flow with a relatively large outside diameter is generated in the central region in the axial direction of the tumble flow by setting the depression width larger. For example, in JP-A-11-218026 and JP-A-7-119472, a large tumble flow is generated in the central region in the axial direction of the tumble flow by forming the outer peripheral shape of the depression as viewed in plan with curved lines.
On the other hand, a part of the tumble flow generated in both outer areas in the axial direction (on both sides of the axis) of the tumble flow (a tumble flow generated at both ends of the piston top surface, which is hereinafter referred to as “side tumble flow”) is significantly influenced by the cylinder bore inner wall existing adjacently in the axial direction of the tumble flow. The side tumble flow influenced by the cylinder bore inner wall includes a flow in the direction perpendicular to the tumble flow axis (a flow generally parallel to the flow line of the center tumble flow) and a flow toward the center side of the piston top surface due to the influence of the cylinder bore inner wall.
FIG. 12 is a plan view of a piston top surface having a depression “a” of a typical shape, in which the directions of the center tumble flow and the side tumble flow flowing in the vicinity of the surface of the depression are indicated by the arrows. In the drawing, “SE” represents a center tumble flow. “SA1” and “SA2” represent a side tumble flow, with “SA1” indicating a flow in the direction perpendicular to the tumble flow axis and “SA2” indicating a flow toward the center side of the piston top surface due to the influence of the cylinder bore inner wall. Also in FIG. 12, “b” indicates the position of an intake valve, while “c” indicates the position of an exhaust valve, respectively.
The energy given to the air (or air-fuel mixture) flowing into the cylinder from the intake port is evenly determined according to the cylinder bore diameter, the piston moving speed, and so forth. That is, the fluid energy of the entire tumble flow is even. Therefore, an effective tumble flow is generated by utilizing the entirety of the given energy to create a flow in the direction perpendicular to the tumble flow axis without a loss.
However, some of the given energy is consumed to create a flow toward the center side of the piston top surface, with the tumble flow actually generated, in particular the side tumble flow, influenced by the cylinder bore inner wall as described above (see the side tumble flow SA2 in FIG. 12). That is, a part of the energy is consumed to create a flow other than in the direction perpendicular to the axis of the tumble flow.
Thus, in the case where such a flow toward the center side of the piston top surface is created, a sufficient flow in the direction perpendicular to the axis of the tumble flow cannot be obtained. In addition, the tumble flow toward the center side of the piston top surface and the center tumble flow interfere with each other, which wastefully consumes the fluid energy of the center tumble flow.
In order to prevent generation of such a flow toward the center side of the piston top surface, other configurations are conceivable in which a depression in a piston top surface is not formed in the vicinity of the cylinder bore inner wall. An example of such configurations is shown in FIG. 13 (a plan view of a piston top surface).
With such a configuration, however, agitation cannot be performed in the cylinder in both outer areas in the axial direction of the tumble flow (regions “d” hatched in FIG. 13), making it difficult to promote sufficient evaporation and atomization of fuel.